Ring-opening Metathesis Polymerization

by Paul Boothe on September 17, 2009

Ring-opening metathesis polymerization (ROMP) uses metathesis catalysts to generate polymers from cyclic olefins. ROMP is most effective on strained cyclic olefins, because the relief of ring strain is a major driving force for the reaction – cyclooctene and norbornenes are excellent monomers for ROMP, but cyclohexene is very reluctant to form any significant amount of polymer. Norbornenes are favorite monomers for ROMP, as a wide range of monomer functionalities are easily available through Diels-Alder reactions.

Careful balance of catalyst, monomer, and other factors can offer excellent control of the polymer structure. In terms of homogeneous catalysts, most tungsten and molybdenum catalysts (Schrock catalysts) have rapid initiation rates and can produce “living” polymerizations with excellent control of polydispersity and chain tacticity, but the low functional group tolerance limits the monomers available. Ruthenium metathesis catalysts (Grubbs catalysts) tend to have slower initiation rates, often leading to higher polydispersities, but their air stability and greater tolerance for functional groups makes them “user friendly” and enables use of a wide range of functional monomers and additives.

Secondary metathesis reactions (controlled by catalyst choice and reaction conditions) also affect the product distribution. Recoordination of an alkene on the growing polymer chain with the catalyst can lead to cyclic oligomers through a ring-closing metathesis reaction (“backbiting”). Chain transfer (cross metathesis) between a growing polymer unit and an adjacent polymer alkene also leads to broadened molecular weights. Chain transfer can also be used to improve processability of the resulting polymer – addition of an acyclic olefin (chain-transfer agent) can limit chain molecular weights and introduce terminal functional groups.

Ring-opening metathesis polymerization has achieved some commercial success, with a variety of ROMP polymers available on the market: Vestenamer® (ROMP of cyclooctene), Norsorex® (ROMP of norbornene), and numerous commercial products from the ROMP of dicyclopentadiene (Telene®, Metton®, Prometa®, Pentam®). Dicyclopentadiene is particularly well-suited to commercial ROMP, as the monomer contains two double bonds of unequal reactivity – a strained norbornene bond that undergoes rapid olefin metathesis, and a cyclopentene bond that can ring-open depending on polymerization conditions to give a cross-linked polymer.